1. Field of the Invention
The present invention relates to a nonvolatile solid memory for storing information, and more particularly to an MRAM using a magnetic material.
2. Related Background Art
In general, a magnetic material such as a ferromagnetic material or a ferrimagnetic material has such a property that magnetization generated in the magnetic material by a magnetic field applied from the outside remains even after the external magnetic field is removed (this is called residual magnetization). Besides, the electric resistance of the magnetic material is changed according to the direction of the magnetization or the existence of the magnetization. This is called a magneto-resistance effect, and the rate of change in the electric resistance value at that time is called an MR ratio (Magneto-Resistance Ratio). As a material having a large MR ratio, there is a GMR (Giant Magneto-Resistance) element or a CMR (Colossal Magneto-Resistance) element, and these are made of metal, alloy, compound oxide, or the like. For example, the material includes Fe, Ni, Co, Gd, Tb, alloys of these, and compound oxide such as LaxSr1-xMnO9 or LaxCa1-xMnO9. If the residual magnetization of the magneto-resistance material is used, a nonvolatile memory can be formed which stores information by selecting an electric resistance value according to the direction of the magnetization or the existence of the magnetization. Such a nonvolatile memory is called an MRAM (Magnetic Random Access Memory).
Most MRAMs under development in recent years use, as a memory element, a magneto-resistance effect element having such a structure that a non-magnetic layer is sandwiched between magnetic layers, and adopt a system in which stored information is read out by converting a change of an electric resistance value, which is caused by a difference in the magnetization direction, into a voltage. Besides, information can be written and can be rewritten by causing a current to flow to a writing wiring and changing the magnetization direction of a memory cell by an induced magnetic field.
The readout of information of the conventional MRAM is such that a current is caused to flow to the memory element storing the information with the residual magnetization of the magnetic material and the resistance value is converted into a voltage so that the information is read out. In this information readout method, a delay occurs by the magneto-resistance element functioning as a resistance. This delay is a main cause of lowering the readout speed of the MRAM. Thus, a large scale MRAM chip has a tendency that the readout speed is low as compared with a synchronous DRAM or SRAM.
Besides, as another information readout method of the conventional MRAM, there is a differential detection method in which in order to read out information stored in one memory cell, a resistance state of a memory cell is changed, and a readout operation is carried out for the same memory cell twice before and after that. In this case, the influence by the delay of the memory element becomes more remarkable. In the present circumstances, a method of carrying out the readout at high speed in such an MRAM has not been achieved.
The present invention has been made in view of the unsolved problems of the related art as stated above, and has an object to provide an MRAM in which the information readout speed of the MRAM is increased up to a speed comparable to a synchronous DRAM, and an information reproducing method of the same.
In order to achieve the above-mentioned object, according to the present invention, there is provided an MRAM comprising a plurality of units each including: plurality of memory elements arranged in a matrix form, each of which includes a non-magnetic layer sandwiched between a hard layer made of a magnetic material and a soft layer made of a magnetic material having coercive force lower than the hard layer; plurality of bit lines arranged in parallel with each other; and plurality of sense amplifiers connected to the respective bit lines, in which the plurality of sense amplifiers in the same unit are activated at the same time to read out information in the unit, the units are successively changed over in synchronization with a clock pulse, and the sense amplifiers in the different units are successively activated, so that information in the plurality of units is parallel outputted in synchronization with the clock pulse, and information of each of the units is continuously reproduced.